全文获取类型
收费全文 | 2553篇 |
免费 | 50篇 |
国内免费 | 16篇 |
专业分类
化学 | 1814篇 |
晶体学 | 29篇 |
力学 | 46篇 |
数学 | 447篇 |
物理学 | 283篇 |
出版年
2022年 | 16篇 |
2021年 | 20篇 |
2020年 | 43篇 |
2019年 | 40篇 |
2018年 | 30篇 |
2017年 | 19篇 |
2016年 | 54篇 |
2015年 | 62篇 |
2014年 | 57篇 |
2013年 | 85篇 |
2012年 | 141篇 |
2011年 | 154篇 |
2010年 | 83篇 |
2009年 | 74篇 |
2008年 | 147篇 |
2007年 | 122篇 |
2006年 | 130篇 |
2005年 | 134篇 |
2004年 | 117篇 |
2003年 | 124篇 |
2002年 | 86篇 |
2001年 | 43篇 |
2000年 | 40篇 |
1999年 | 24篇 |
1998年 | 23篇 |
1997年 | 34篇 |
1996年 | 30篇 |
1995年 | 29篇 |
1994年 | 27篇 |
1993年 | 19篇 |
1992年 | 21篇 |
1991年 | 24篇 |
1990年 | 19篇 |
1989年 | 20篇 |
1988年 | 27篇 |
1987年 | 32篇 |
1986年 | 21篇 |
1985年 | 46篇 |
1984年 | 39篇 |
1983年 | 43篇 |
1982年 | 50篇 |
1981年 | 39篇 |
1980年 | 41篇 |
1979年 | 23篇 |
1978年 | 31篇 |
1977年 | 20篇 |
1976年 | 20篇 |
1975年 | 22篇 |
1974年 | 16篇 |
1973年 | 15篇 |
排序方式: 共有2619条查询结果,搜索用时 15 毫秒
11.
We consider an Ising model with Kac potential dK(¦x¦) which may have arbitrary sign, and show, following Gates and Penrose, that the free energy in the classical limit0+ can be obtained from a variational principle. When the Fourier transform of the potential has its maximum atp=0 one recovers the usual mean-field theory of magnetism. When the maximum occurs forp
00, however, one obtains an oscillatory or helicoidal phase in which the magnetization near the critical point oscillates with period 2/¦p
0¦. An example with a potential possessing parameter-dependent oscillations is shown to exhibit crossover phenomena and a multicritical Lifshitz point in the classical limit. 相似文献
12.
The role of search theory in the exploitation of natural resources is discussed in this paper. After a brief history and taxonomy of search problems, the mathematics of search is discussed. This includes underlying probability distributions, the differential equations of search, Bayesian use of search information and optimization problems in search theory. The theory is illustrated by applications in fisheries, pest control, animal foraging, and oil and mineral exploration. 相似文献
13.
Relative rates of solvolysis of some N-triorganosilylanilines in mixtures of ethanol and aqueous potassium hydroxide have been determined, with results as follows. (i) For XC6H4NHSiEt3 compounds in MeOH (5 vol) + aq. alkali (2 vol) at 50°: (X =) H, 1.0;p-Me, 0.80;p-OMe, 0.83;m-Me, 0.90; o-Me, 0.87; p-SMe, 1.90; p-F, 1.7; p-Cl, 2.8; o-Cl, 14; m-Cl, 4.2; m-NO2, 18; p-CN, ca. 43; p-NO2, ca. 120. (ii) For PhNHSi(C6H4Y)3 compounds in MeOH (10 vol) + aq. alkali (1 vol) at 50°: (Y =) H, 1.0; p-OMe, 0.12; p-Cl, ca. 32; m-Cl, ca. 84. (iii) For PhNHSiR3 compounds in MeOH (5 vol) + aq. alkali (2 vol) : (R3 =) Et3, 1.0; Et2Me, 18 (at 30°); Me2-i-Pr, 8 (at 30°);Me2-t-Bu, 0.012 (at 50°);i-Pr3, 0.006 (at 50°). In series (i) the relative rates correlate with σ, or where appropriate σ?-constants, with a ? value of 1.6. It is suggested that in the transition state of the rate-determining step the OSi bond is fully formed, or almost so, the SiN bond approximately 20—50% broken, and the bond between the nitrogen atom and a proton from the solvent ca. 10—30% formed. 相似文献
14.
Tetra-n-butylammonium hexachloroplatinate (IV) reacts with lithium methyl/lithium iodide in ether to give a solution containing lithium hexamethylplatinate (IV). With lithium methyl/lithium bromide in ether however, tetrabutylammonium hexamethylplatinate (IV) is precipitated together with lithium halides. Solid [Bu4N)2[Pt(Ch3)6] is stable under nitrogen at room temperature, but ether solutions of [Pt(Ch3)6]2- decompose in a few minutes at room temperature in the absence of excess lithium methyl. 相似文献
15.
Colin Eaborn 《Journal of organometallic chemistry》1985,290(3):c43-c44
Heterobinuclear complexes of formula [LMCl2(pz)M′(tfb)] (M = Ru, L = p-cymene, M′ = Rh; M = Ir, L = C5Me5, M′ = Rh; M = Rh, L = C5Me5, M′ = Ir) and [(C5Me5)IrCl(pz)2Rh(tfb)] (tfb = tetrafluorobenzo[5.6]bicyclo[2.2.2]octan-2,5,7-triene) have been prepared. The molecular structure of [(p-cymene)Ru(μ-Cl)2(μ-pz)Rh(tfb)] has been determined by X-ray diffraction. It consists of two moieties, (p-cymene)Ru and (tfb)Rh, triply-bridged by a pyrazolate group and two chlorine atoms. 相似文献
16.
Geneviéve Cerveau Ernesto Colomer Robert J.P. Corriu J.Colin Young 《Journal of organometallic chemistry》1981,205(1):31-45
We report the chemical behavior of cobalt-stabilized carbenes, R3M(CO)3CoC(OEt)R′, and their parent anions, R3M(CO)3CoC(O?)R′, where M = Si or Ge. The anions where M = Si, R′ = Ph decompose thermally into the corresponding benzoylsilanes; when the silicon atom is chiral (R3 = MePh-1-Np) optically active R3SiCOPh is obtained with complete retention of configuration. 相似文献
17.
Conventional figures of merit such as limits of detection, signal to background ratio or repeatability, are used to determine the performance of pneumatic and ultrasonic sample introduction systems in an ultra-clean environment with an axially viewed inductively coupled plasma-atomic emission spectrometry and multichannel detection. We observed that the ultrasonic nebuliser offered a large improvement of signal intensity (10-133 greater) compared to a cyclone chamber coupled with a pneumatic Meinhard nebuliser. This improvement is associated with an average increase of signal to background ratio by a factor 86 and an average decrease of detection limits by a factor 6. The improvement factors generally depend on the element and for the same element on spectral lines. Typically, the observed values of detection limits in this work are lower than those published and obtained in non-ultra-clean conditions. The results emphasize that the environmental conditions of cleaning and analysis are essential to avoid and control cross contamination of the samples and hence to obtain low detection limits. 相似文献
18.
Tsai P Ichikawa K Mailer C Pou S Halpern HJ Robinson BH Nielsen R Rosen GM 《The Journal of organic chemistry》2003,68(20):7811-7817
Apparent rate constants, at acidic pH and neutral pH for the reaction of a family of ester-containing 5-carboxyl-5-methyl-1-pyrroline N-oxides with superoxide (O2*-) were estimated, using ferricytochrome c as a competitive inhibitor. It was of interest to note that the rate constants were similar among the different nitrones and not that significantly different from that found for 5-(diethoxyphosphoryl)-5-dimethyl-1-pyrroline N-oxide. At acidic pH, the rate constant for spin trapping O2*- was 3-fold greater than that at physiological pH. Subsequent experiments determined the half-life of aminoxyls, derived from the reaction of these nitrones with O2*-. The EPR spectra were modeled by using a global analysis method. The results clearly demonstrated that EPR spectra of all the aminoxyls were inconsistent with a model that included a single gamma-hydrogen splitting. A better interpretation modeled them as two diastereomers with identical nitrogen splittings and slightly different beta-hydrogen splittings. Detailed line width analyses slightly favored an equal line width-unequal population ratio for the two diastereomers. 相似文献
19.
Decafluorocyclohexene reacted slowly with aniline to give 1-phenylamino- 3-phenyliminoheptafluorocyclohex-2-ene, which was hydrolysed by hydrochloric acid to 3-phenylaminoheptafluorocyclohex-2-enone. Decafluorocyclohexene reacted stepwise with phenyl lithium, giving 1-phenylnonafluorocyclohexene and thence 1,2-diphenyloctafluorocyclohexene: the former product was attacked slowly by pentafluorophenyl lithium at ?40°C affording 1-pentafluorophenyl- 1-phenyloctafluorocyclohexene. Phenyl lithium reacted sluggishly with bis(pentafluorophenyl)octafluorocyclohexene to give 1-pentafluorophenyl-2-(2′,3′,5′,6′-tetrafluoro-1′-biphenylyl)octafluorocyclohexene and 1,2-bis(2′,3′,5′,6′-tetrafluoro-1′-biphenylyl)octafluorocyclohexene. 1,2-Diphenyloctafluorocyclohexene and 1,2-bis(pentafluorophenyl) octafluorocyclohexene were fluorinated by cobalt(III) fluoride to give the olefin, 1,2-bis(undecafluorocyclohexyl)octafluorocyclohexene. 相似文献
20.
Uracil and thymine react with benzoyl chloride in acetonitrile-pyridine solution at room temperature to give first their 1--benzoyl (2b and 3b) and then their 1-, 3--dibenzoyl derivatives (4a and 4b, respectively); the latter compounds are converted into the corresponding 3--benzoyl derivatives (5a and 6a) under mild conditions of basic hydrolysis. 相似文献